Carver's "Neighbors" is a story about voyeurism. Voyeurism is normally associated with gaining sexual pleasure from watching sex acts performed by others. Notice how the husband's visits to the neighbors' apartment become sexual for him. Her visits probably have the same effect on the wife, too--she is the one who finds the pictures and brings them to her husband--but we don't know for sure because the camera-like narrator only follows the husband.
But voyeurism is more than just watching, etc. The couple become fascinated with and obsessed by the neighbors' lives, to the extent that, again, being in the apartment becomes an erotic experience in itself.
In a twist to the voyeurism theme, the husband, while left alone in the apartment behind closed doors, if you will, though experiencing pleasure from being a part of the lives of others, behaves in ways that he apparently only does in private.
In another twist, the eroticism engendered in private, carries over to his marriage. This couple is not falling apart--they're having the time of their lives.